[budman]

If there are three people in the hand and you are second to bet on the river and you check out, then the third person is given an invitation to bluff at the pot.

In the worst case, if your ten high was good, then it is highly unlikely that the third person would bluff at the pot with two possible callers and such a poor hand.

In this case if you were player one with a pair of fours you would certainly appreciate the second player 'protecting your hand' by at least not folding for free.

I think it is more a courtesy issue than ethical.